JSON mostly won the API wars, but XML hasn't gone anywhere — it still runs a huge amount of enterprise infrastructure, document formats, and configuration. Picking between them isn't about which is "better"; it's about which features you actually need.
The core difference
JSON describes data: objects, arrays, strings, numbers, booleans, null. That's the entire type system. It's compact and maps directly onto data structures in JavaScript, Python, and most other languages, which is why it's the default for REST APIs.
XML describes documents, and data is one thing documents can contain. Every XML element can carry attributes alongside its content, supports comments, allows mixed content (text and elements interleaved), and can be validated against a formal schema (XSD or DTD) before anything touches your code. None of that exists in JSON.
Where JSON wins
- APIs and microservices. Smaller payloads, no closing tags to repeat, and native parsing in virtually every language and browser.
- Configuration files for apps and tooling. package.json, tsconfig.json — the entire JavaScript ecosystem defaults to it.
- NoSQL and document databases. JSON (or a binary variant like BSON) is usually the native storage format.
You can check any JSON payload for syntax errors with line-and-column precision using our JSON Validator, or clean up minified API responses with the JSON Formatter.
Where XML still wins
- Document formats. DOCX, XLSX, SVG and RSS are all XML under the hood — because documents need namespaces, mixed content and attributes in ways plain data structures don't.
- Strict schema validation. If a contract between systems needs to be formally verified — financial messaging (SWIFT, FIX-adjacent formats), healthcare (HL7), government data exchange — XSD gives you validation JSON Schema only approximates.
- Legacy enterprise systems. SOAP, many banking and insurance back ends, and a lot of enterprise Java infrastructure were built around XML and haven't moved, often for good reason: the switching cost is real and the systems work.
Converting between the two
There's no lossless, automatic conversion between arbitrary JSON and XML — JSON has no equivalent of attributes or mixed content, so a converter has to make assumptions about how to represent them. In practice, most "JSON to XML" conversion is really just mapping a specific, known JSON shape onto a specific, known XML shape by hand, rather than a generic transformation.
What you can do generically is validate and format each format on its own terms — catch a missing comma in JSON, or an unclosed tag in XML — before whatever downstream process consumes it. That's what our XML Formatter and JSON Formatter are for: not conversion, but making sure the document you have is actually well-formed.
If you're starting something new
Default to JSON unless you have a specific reason not to: you need attributes, mixed content, namespaces, or formal schema validation that JSON Schema doesn't adequately cover for your case. For a plain data payload — the overwhelming majority of new APIs — JSON's simplicity is the right trade-off.